Researchers have debated for more than two decades the likely impacts, if any, of global warming on the worldwide incidence of malaria, a mosquito-borne disease that infects more than 300 million people each year.

Now, researchers from the London School of Hygiene & Tropical Medicine and the University of Michigan, with colleagues, are reporting the first hard evidence that malaria does—as had long been predicted—creep to higher elevations during warmer years and back down to lower altitudes when temperatures cool.

The study, due to be published in Science and based on records from highland regions of Ethiopia and Colombia, suggests that future climate warming will result in a significant increase in malaria cases in densely populated regions of Africa and South America, unless disease monitoring and control efforts are boosted and sustained.

Menno Bouma, Honorary Senior Clinical Lecturer at the London School of Hygiene & Tropical Medicine, said: "Traditionally, we think of malaria as a disease with limited prevalence in highland regions, but we are now seeing a shift due to climate change. Our latest research suggests that with progressive global warming, malaria will creep up the mountains and spread to new high-altitude areas. And because these populations lack protective immunity, they will be particularly vulnerable for severe morbidity and mortality."

This latest study is the culmination of research which began over 20 years ago at the London School of Hygiene & Tropical Medicine, when malaria and climate data was first collected in the Debre Zeit area of Ethiopia.

It had been predicted that malaria as a disease could be especially sensitive to climate change, because both the Plasmodium parasites that cause it and the Anopheles mosquitoes that spread it thrive as temperatures warm. But some of the assumptions behind those predictions were criticised, and more recently, some researchers have argued that improved socioeconomic conditions and more aggressive mosquito-control efforts will likely exert a far greater influence over the extent and intensity of malaria worldwide than climatic factors.

What's been missing in this debate has been a detailed analysis of regional records to determine how the spatial distribution of malaria cases has changed in response to year-to-year temperature variations, especially in countries of East Africa and South America with densely populated highlands that have historically provided havens from the disease.

Bouma and his colleagues looked for evidence of a changing spatial distribution of malaria with varying temperature in the highlands of Ethiopia and Colombia. They examined malaria case records from the Antioquia region of western Colombia from 1990 to 2005 and from the Debre Zeit area of central Ethiopia from 1993 to 2005.

By excluding other variables that influence malaria case numbers, such as mosquito-control programs, resistance to anti-malarial drugs and fluctuations in rainfall amounts, they found that the median altitude of malaria cases shifted to higher elevations in warmer years and back to lower elevations in cooler years. The relatively simple analysis yielded a clear, unambiguous signal that can only be explained by temperature changes, they said.

In the Debre Zeit region of Ethiopia, about 37 million people live in rural areas at risk of higher malaria exposure under a warming climate. In a previous study, researchers estimated that a 1 degree Celsius temperature increase could result in an additional 3 million malaria cases annually in Ethiopian children.

"This is indisputable evidence of a climate effect," said Mercedes Pascual from the University of Michigan. "Our findings here underscore the size of the problem and emphasize the need for sustained intervention efforts in these regions, especially in Africa."

The authors note that their work is limited to two countries on two continents, and should be replicated in more countries with malaria in highland regions before more general trends can be assumed.